Education officials are trying to determine what led a former computer teacher at the New Millennium Business Academy in the Bronx to barricade himself Friday, prompting a three-hour police standoff.

Francisco Garabitos, 55, was told not to come back to work after being investigated for allegedly hitting a student Thursday. When Garabitos showed up at school Friday morning, the principal called 911 and students were evacuated.

"We know that there has been a history of problems at this school," says Randi Weingarten, president of the United Federation of Teachers.

Garabitos, the UFT chapter leader of the school, apparently had several conflicts with Principal Dorald Bastian, who had given Garabitos two unsatisfactory ratings in the past.

"We know that there was a bad relationship between the principal of the school and himself, as well as [with] several of the other employees in the school," Weingarten says.

News 12 the Bronx obtained an exclusive copy of a letter Garabitos sent to District Superintendent Dolores Esposito, listing the grievances he and 20 other teachers had with Bastian. In the letter, Garabitos wrote that the principal engaged in professional harassment, used anti-union language and held personal grudges. The letter went on to say that a way to solve the problem would be to terminate or reassign Bastian and the assistant principal.

Weingarten says Garabitos' issues with the management of the school have not been addressed, possibly leading to his actions Friday.